KURDISH-CONTROLLED NORTHERN IRAQ (CNN) --In one of the largest airborne infantry operations in decades, U.S. paratroopers secured an airfield in northern Iraq early Thursday, while Baghdad continued to come under air attack from the U.S.-led coalition.

In a chilling development, a top U.S. official said Wednesday night that Iraqis had executed some prisoners of war. (Full story)

Earlier in the day, Pentagon sources told CNN they were looking into a report that Iraqi soldiers shot dead seven U.S. Army soldiers as they were surrendering with their hands up Sunday.

In Thursday's airborne drop, about 1,000 U.S. paratroopers from the U.S. 173rd Airborne Brigade parachuted into Kurdish-controlled northern Iraq under the cover of darkness, military officials said.

The officials characterized the region where the drop occurred as "semi-permissive," meaning it was not hostile.

An airlift of elements of the U.S. 1st Infantry Division, equipped with Abrams tanks and Bradley fighting vehicles, followed the airborne drop and was in progress. (Full story)

The U.S. military had hoped to transport vehicles and troops overland, but failed to reach an agreement with Turkey about using its bases and airspace.

Smoke rose over the skies of Baghdad about 3:45 a.m. Thursday (7:45 p.m. ET Wednesday) after a volley of explosions. About an hour earlier, explosions were heard on the city's outskirts.

Live pictures on the Arab-language TV network Al-Jazeera showed a large cloud of smoke rising near a multi-story building.

A pilot returning from a bombing run to the USS Constellation told a CNN reporter on the carrir that an Iraq airfield near Baghdad where MiG fighters are based was among the targets.

Iraqi troop movements

Meanwhile, in central Iraq, there were conflicting reports about whether large numbers of Iraqi Republican Guard troops were moving out of Baghdad toward the lead elements of the U.S. 3rd Infantry Division.

Top U.S. military officials in Washington and at Central Command headquarters in Qatar said field reports seemed to be based on inaccurate intelligence and that officials could find no evidence of such an operation.

CNN Correspondent Walter Rodgers, accompanying the 3rd Squadron, 7th Cavalry -- the 3rd Infantry Division's reconnaissance unit -- said officers told him a column of up to 1,000 Iraqi vehicles was probably heading south to try to retake a key bridge over the Euphrates River. The bridge was taken by U.S. forces in an intense battle early Tuesday.

Farther south Wednesday, a column of up to 120 Iraqi armored vehicles poured out of the city of Basra, British reporters told CNN. British artillery and U.S. warplanes were attacking the Iraqis as they moved southeast toward the Al Faw Peninsula.

Basra was the scene of a reported uprising against the Iraqi government Tuesday, and an Iraqi Shia Muslim leader called the situation there "very, very unstable."

"Yesterday, some streets went into uprising, but they were immediately repressed by the people of the regime," said Ayatollah Sayed Mohamad Baqir Al-Hakim, leader of the Iran-based Supreme Council for Islamic Revolution in Iraq.

"The allied forces are still surrounding Basra, and they entered into some northern areas of Basra."

Reporters stationed in Basra from Abu Dhabi television and Al-Jazeera satellite network said they saw no signs of civilian unrest. (Full story)

Market bombing dispute

In Washington, British Prime Minister Tony Blair arrived for talks with President Bush at Camp David. (Full story)

Earlier, Bush warned that while the war "is far from over," the "day of reckoning for the Iraqi regime ... is drawing near."

"As they approach Baghdad, our fighting units are facing the most desperate elements of a doomed regime," Bush told troops in a speech at MacDill Air Force Base in Tampa, Florida -- home of U.S. Central Command. (Full story)

At the United Nations, Iraq's U.N. ambassador, Mohammed Aldouri, told the Security Council "the Iraqi people are being subjected to a criminal and barbaric American-British military invasion" that has led to "thousands of casualties, among them women, children, and the elderly."

From Baghdad, Iraqi Information Ministry officials said U.S. munitions killed 15 Iraqi civilians Wednesday at a popular market.

Photographers from international news agencies confirmed seeing an undetermined number of dead and injured people, apparently civilians, and burned vehicles in the capital city.

Before the incident, Iraq had reported 78 civilian deaths since Monday as a result of bombing raids. CNN could not independently verify Iraq's figures.

At the Pentagon, Maj. Gen. Stanley A. McChrystal denied coalition forces targeted the marketplace, but he could not say whether the destruction there was the result of Iraqi weapons or U.S. missiles that went astray. He suggested the Iraqis firing at allied aircraft could be responsible for the casualties.

"Another explanation could be the triple-A fire [anti-aircraft artillery] or surface-to-air missiles that missed their targets and fell back into the marketplace area," he said.(Full story)

Fury of the Fedayeen

Meanwhile, U.S. war planners may have miscalculated the strength and capability of paramilitary fighters in Iraq, a Pentagon official said Wednesday.

Fighters of the Fedayeen Saddam, one of Iraq's most feared paramilitary groups, along with members of the ruling Baath Party and other security forces, have launched several attacks against U.S. and British forces for a week.

"We may have underestimated that they were dispersed to so many places across Iraq to enforce regime discipline," the Pentagon official said. "It's clear now they dispersed sometime before the war began."

Members of the group have caused problems for coalition forces in some areas, where they have disguised themselves as civilians to ambush troops or pretended to surrender and opened fire -- acts the United States have called "serious violations of the laws of war." (Full story)

Retired Gen. Wesley Clark, a CNN analyst and former NATO supreme allied commander, said the scenario of a quick victory was "not going to happen." (Full story)

Other developments

• British Marines found a large cache of hand grenades, rocket-propelled grenades and Iraqi military uniforms stored in three classrooms of a school in the southern Iraqi port city of Umm Qasr, a British pool reporter said Wednesday. The uniforms may indicate the estimated 120 Iraqi troops there discarded their military clothes and "melted away into the civilian population," the reporter said. (Full story)

• A survey by the Pew Research Center found that 72 percent of Americans believe that using military force in Iraq was the right thing to do. The poll was conducted March 20-24, and it found that support for the war has remained fairly steady, even though the percentage of people who said the war was going "very well" dropped from 71 percent Friday and Saturday to 38 percent Monday. Researchers interviewed 1,495 people for the survey, which had a margin of error of plus or minus 3 percentage points.

• Seven battered transport trucks carrying about 12 tons of food, medical supplies and other goods Wednesday pulled into Umm Qasr. The British were working with humanitarian organizations on the best ways to distribute the aid. (Full story)

• A second U.S. soldier has died of wounds he suffered in a grenade and small-arms attack at his base in Kuwait. A fellow soldier is in custody in Germany in connection with the attack pending a pretrial investigation. (Full story)

• Forty-seven U.S. and British military personnel have been confirmed killed since the conflict began. (Coalition casualties)